The present invention relates to a method for molding plastics and other materials. More specifically, the present invention relates to a molding method suitable for application in molding materials having poor fluidity or thermal stability, which typically can include plastic materials such as polyethylene and others having ultra-high molecular weight, or mixtures of plastic materials such as polyvinyl chloride and non-plastic materials such as wood flour, paper powders, stone powders, metal powders, fiber reinforcements, and the like.
In the field of technologies for molding plastics, numerous improvements have been proposed and implemented to bring about remarkable progress in the related area.
In Japanese Patent Application Laid-Open No. 190891/1994, the present inventor has proposed a molding technique for plastic materials wherein the production rate and quality of the molded products can be improved by increasing bulk density of the raw material within the extrusion cylinder.
On the other hand, in line with requirements related to diversification and recycled use of plastic moldings in recent years, new products have emerged and are used in applications such as building materials, examples of which include molded products of wood-like appearance using raw materials comprising mixtures of plastic materials and wood flour, or molded products of marble-like appearance using raw materials comprising mixtures of plastic materials and marble stone powders.
In the prior art, however, molded products having wood-like appearance comprising mixtures of plastic materials and wood flour, or those comprising mixtures of plastic materials and marble stone powders lacked the appearance and tactility of genuine wood or marble and therefore were unable to offer higher commercial values because the mixing ratios of wood flour or stone powders to the plastic materials were limited typically to a level less than 20 to 30% by weight.
The reason that the mixing ratios of wood flour or stone powders must be limited to a level less than 20 to 30% by weight as above is that, raw materials comprising plastic materials mixed with 20 to 30% by weight or more of non-plastic materials such as wood flour or stone powders typically exhibit reduced fluidity and thermal stability when an extrusion molding is performed on them. This in turn causes an overheating of the non-plastic materials contained in the raw material, leading to carbonization and a phenomenon so-called xe2x80x9cburnxe2x80x9d in the molded products. This phenomenon can occur in a similar fashion on raw materials comprising plastic materials mixed with more than 30% by weight of paper powders.
Meanwhile, in the case of raw materials having poor fluidity or thermal stability, the molded products sometimes develop cavities within them (i.e. xe2x80x9ccavitationxe2x80x9d), even though there were no burn-related problems.
Such phenomena as burn or cavitation above have also been experienced when a plastic material of poor fluidity or thermal stability is used alone.
Furthermore, when a molding operation is performed on materials mixed with 5% by weight or more of stone powders or those mixed with more than several percent by weight of metal powders using a conventional extruding machine, wear on the extrusion screw can become excessive, particularly at its tip portion, requiring a frequent replacement of the screw at an increased cost of operation.
As a result of extensive research conducted with a view to eliminating aforementioned problems, the present inventor has found that a smooth and high quality molding operation can be made available for plastic materials of poor fluidity or thermal stability, or for raw materials based on plastic materials mixed with non-plastic materials such as wood flour, paper powders, stone powders, metal powders, or fiber reinforcements at ratios higher than that conventionally applied, by carrying out melt-kneading of raw materials under oxygen-free conditions, shortening melt-kneading time duration, feeding raw materials from the screw tip in straight flow conditions, and/or reducing the shearing force working at the screw tip material.
The present invention has been made in this situation and has an object of providing a method for performing satisfactory molding for plastics having poor fluidity or thermal stability or for plastic materials mixed with more than 10 to 30% by weight of wood flour, paper powders, stone powders, metal powders, fiber reinforcements, and the like, without causing molding defects such as burn or cavitation, and also for raw materials mixed with more than several percent by weight of metal powders or stone powders without causing excessive wear at the extrusion screw.
In accordance with the method for molding plastics and other materials of the present invention, a raw material comprising a plastic material or a mixture of plastic and non-plastic materials is fed into a cylinder wherein said raw material is subjected to a melt-kneading performed under oxygen-free conditions.
One example of the possible methods for providing oxygen-free conditions for a raw material comprises feeding the raw material from a hopper into a cylinder through a valve, a connecting tube, and an opening to the cylinder while the raw material is subjected to deaeration as it travels through the path from said valve to said cylinder and, at the same time, effecting an additional deaeration in the vicinity of the opening to said cylinder with a suction force equal to or greater than that applied for the deaeration provided in the path from said valve to said cylinder, thereby achieving the desired oxygen-free conditions for the raw material. It is more desirable if the above hopper is hermetically sealed and, furthermore, feeding of raw materials to said hopper is provided alternately from a plurality of sub-hoppers that are also hermetically sealed.
Since the above-described method helps hinder oxidative phenomena occurring within the cylinder, carbonization of the raw material can be minimized and therefore the burn defects in the molded products can be avoided.
Furthermore, in the method for molding plastics and other materials according to the present invention, it is desirable to feed a molten raw material from the tip of the screw to the head section of the cylinder while maintaining straight flow conditions. As a specific means to feed a raw material to the head section of the cylinder in straight flow conditions, use of a screw having a tip angle of from 30 to 120 degrees can be given, by way of an example.
An arrangement for the screw tip angle such as described above enables to eliminate burn defects occurring due to a part of the molten raw material being trapped in the swirl generated around the screw tip and flowing stagnantly, or cavitation defects caused due to starvation of the molten raw material occurring otherwise in the area around the center of the screw tip.
Moreover, to feed a molten resin to the head section of the cylinder in better straight flow conditions, it is desirable to reduce the shearing force working on the molten resin.
In the present specification, the passage xe2x80x9coxygen-free conditions for a raw materialxe2x80x9d refers to the conditions wherein gaseous substances such as air are removed from the raw material itself as well as from the interspaces held in the raw material bulk, thereby leaving only a small amount of oxygen remaining in it.
Also in the present specification, the word xe2x80x9cmoltenxe2x80x9d can sometimes refer to a semi-molten state in addition to a completely molten state.
Furthermore, in the present specification, the passage xe2x80x9cstraight flow conditionsxe2x80x9d (or so-called xe2x80x9cplug flowxe2x80x9d) is defined as conditions wherein a molten raw material flows along the screw tip with an approximately equal flow velocity across the radial sections and without involving irregular flows such as a turbulent flow.
In the above-mentioned method for molding plastics and other materials, raw materials containing from 10 to 80% by weight of non-plastic materials are also included as the object of molding, as well as raw materials comprising plastic materials in either pellet or powder form and non-plastic, powder form materials, both being fed from hoppers.
The equipment for molding plastics and other materials according to the present invention is provided with a pair of hoppers and a main hopper mounted underneath them, a sealing valve connected to said hopper, a connecting tube placed between said sealing valve and an opening of a cylinder, primary suction holes provided on said connecting tube, a screw inserted in said cylinder, and secondary suction holes provided in the vicinity of the opening at the inner diameter wall of said cylinder, with a ratio of the cylinder length to the screw length being from 1:0.98 to 1:0.75.
In the above arrangement, it is more desirable to configure a plurality of sub-hoppers on top of the above-mentioned hopper, each being connected through a sealing valve.
A configuration such as described above enables feeding of raw materials into the hopper in a continuous mode while avoiding entraining air.